The invention is directed to a single mode optical waveguide fiber wherein a refractive index profile design is optimized for high data rate links, or systems using optical amplifiers, or wavelength division multiplexed systems.
The full capability of optical waveguide fiber is being exploited by high data rate systems having a long distance between repeaters. The operating window in a range including 1550 nm is attractive for these systems because of the lower attenuation possible and the absence of absorption peaks. Data rates typical of such systems are greater than 1 gigabit/sec and repeater spacing exceeds 50 km.
The high data rates require that the birefringence of the waveguide fiber be low. That is, the dispersion of the polarizations of the single propagated mode must be controlled to limit bit errors. The high data rates also require that the zero dispersion wavelength be near 1550 nm to limit material dispersion. Furthermore, the introduction of high powered lasers has produced non-linear effects which can limit data rate or repeater spacing. In systems which utilize wavelength division multiplexing over a relatively small wavelength range, the non-linear interference effect called four wave mixing (FWM) is especially detrimental.
One approach to limiting polarization mode dispersion (PMD) is to provide a waveguide fiber which is relatively free of birefringence. This may be accomplished by maintaining circularly symmetric geometry and by limiting residual stress in the fiber. In addition, a waveguide having a relatively lower dopant level in the signal carrying portion of the waveguide will have reduced Rayleigh scattering and will reduce bit errors due to non-linear effects.
The impact of non-linear effects can also be lessened by providing a larger mode field diameter to reduce power density in the waveguide fiber. Four wave mixing can essentially be eliminated by moving the zero dispersion wavelength out of the operating window. A non-zero dispersion over the operating window serves to prevent the phase matching of multiplexed signals thereby eliminating the four wave mixing signal interference.
The objectives, therefore, in manufacturing a waveguide fiber for high data rate, long repeater spacing and multichannel operation are to provide:
low residual stress; PA1 reduced overlap of signal with higher dopant waveguide regions; PA1 higher modefield; and, PA1 dispersion zero away from the operating window.
Further, these properties must be achieved while maintaining low attenuation, acceptable bend performance and appropriate cut off wavelength. An added benefit can be realized if the performance goals can be met without increasing manufacturing difficulty or cost.